Ligaments are tissues made up of fibrous bands of collagen that join a bone to another bone. Stability and limited excess motion are what ligaments provide joints. The knee contains four main ligaments: the medial collateral (MCL), lateral collateral (LCL), anterior cruciate (ACL) and posterior cruciate (PCL) ligaments. Each of these ligaments performs a specific function in creating stability in the knee. If a ligament is injured, both the function and stability of the knee can be drastically reduced.